This invention relates to hygrometers or humidity sensors of the electrical impedance type which are used for measuring continuously the dewpoint of a gas or gas mixture.
Conventionally the detection and measurement of humidity is performed at a single location using an electrical impedance type of hygrometer. The sensing element of such a hygrometer typically comprises a substrate formed of aluminium on which a porous aluminium oxide film is formed by anodic oxidation. This porous aluminium oxide film is then covered by a porous conducting electrode. Water vapour from the atmosphere surrounding the sensor element is adsorbed onto the porous aluminium oxide layer and its electrical properties such as its dielectric constant and its resistance vary with the quantity of water vapour adsorbed onto it. As the electrical properties of the porous aluminium oxide layer vary so does the impedance of the sensing element. The hygrometer includes a sensing circuit which measures the electrical impedance of the sensing element and produces an output indicative of the absolute, or percentage humidity of the atmosphere surrounding the sensor element. It is also known to use other materials such as silicon for the substrate.
As a result of the hygrometer relying upon a change in the electrical impedance of the sensing element to determine the humidity of the atmosphere surrounding it any change in the electrical characteristics of the sensing element, for example as a result of ageing or corrosion, which is not directly attributable to a change in the humidity will give rise to a flase indication of the humidity.